The phosphor material, manganese doped zinc sulphide, has been incorporated in fine particle powder form as a layer enclosed between electrode bearing substrates. In particular there is a dc electroluminescent panel that incorporates copper coated particles of this material, a material that is activated by a preliminary process of electrical forming. During this process, as the layer becomes heated by the dissipation of primary current, copper migrates away from one of the electrode bearing substrates leaving a thin region of high resistivity, a region depleted of copper. In the subsequent operation of this panel, it is this thin region that serves as the electroluminescent source.
An alternative to this structure, a two layer structure comprising a thin active layer of manganese doped zinc sulphide powder and, in intimate contact with this, a thicker layer of copper coated zinc sulphide powder, is described in GB. Patent No. 1,571,620. Priming by the process of electrical forming, is obviated since both high resistivity and low resistivity regions, two layers, are provided during manufacture.
In both the structures described above, the presence of mobile copper has a stabilising effect. Any anomalously low resistivity part of the high resistivity region that develops, causes localised heating and a migration of copper, resulting in correction of local resistivity.
Higher efficiency, ie better luminance, may be achieved, using instead of powdered phosphor, a relatively thin film of phosphor material for the high resistivity layer. It is however difficult to produce uniform flawless thin film, and device yield and lifetime is low. For example, a pinhole flaw in the film can lead to high localised heating, arcing, and catastrophic disruption of the film. However, attempts to produce manganese doped zinc sulphide film--eg by sputter implantation of manganese in preformed zinc sulphide film--have to date proved ineffectual for dc electroluminescent panel construction.
A conventional ac thin film electroluminescent panel (ACTFEL) is comprised of a thin phosphor film sandwiched between a pair of insulated electrode bearing glass substrates. Thin film ZnS:Mn devices are now in commercial use. Hitherto the favoured methods of depositing thin films of ZnS:Mn have been by sputtering or electron beam (E-beam) evaporation. In both cases a subsequent heat treatment at 450.degree. C. is normally necessary to provide acceptable luminescent film quality. Current state of art devices emit a mean luminance of about 20 ft L, when driven with 0.5% pulses exceeding 200 V peak magnitude. Attempts to reduce drive voltage by making thinner films yield lower (and therefore unacceptable) brightness.